WIP balances can be written off in the same way as Interim Balances.
Log in to Time+Billing.
Select the client, then go to Input, Invoice (WIP).
Create a nil invoice by writing on and/or off the WIP balance, so the total invoice value is zero (0).
Select Post to post the invoice and reflect the changes on the client.
There is no option to write off WIP in bulk. This process needs to be done one by one. A Nil invoice will be required if you want to write the WIP off completely.